1. Field of the Invention.
This invention relates to high-speed network systems and, more particularly, to methods and apparatus for configuring and reconfiguring the operational mode of a station for such a network.
2. Prior Art.
Local area networks (LANS) are increasingly being used in office and factory applications. While several LAN protocols are currently available, applications in parallel processing, industrial control, inter-networking, and real-time voice and video systems require data rates that exceed those currently available.
The X3T9 Committee of the American National Standards Institute (ANSI) has defined a Fiber Distributed Data Interface (FDDI) network standard to provide increased network bandwidth. The Fiber Distributed Data Interface (FDDI) network uses two rings of optical fibers to interconnect up to 500 stations, or nodes to the network. The data counter-rotates on the rings and uses a timed token passing access protocol. The dual ring approach is used to minimize problems due to cable faults and station, or node, failures. The fiber optic rings themselves each consist of a series of point-to-point connections between neighboring nodes, where each node must repeat data for the network to be operating successfully. A primary ring is used for data transmission. A secondary ring (sometimes called the redundant ring) is also used for data transmission but also functions as a backup ring in the event of cable link or station failure. Each of the rings has a bandwidth of 100 Mbits per second.
The FDDI standard specifies four distinct protocol layers:
The Media Access Control (MAC) Layer selectively allocates the right to transmit data in the network among the various stations in a network. The MAC Layer defines a special timed-token protocol which guarantees efficient transmission of data by ensuring that a particular station can transmit a minimum amount of information on the ring within a predictable amount of time.
The Physical Protocol (PHY) Layer defines a groupencoding algorithm called 4B/5B and an elasticity buffer to maintain data synchronization between the network and a station.
The Physical Media Dependent (PMD) Layer defines the optical cable, transmitters, receivers and connectors used for implementation of the standard.
The Station Management (SMT) Layer defines bandwidth allocation and fault isolation methods; coordinates activity of the PMD, PHY, and MAC layers within a station; and manages neighboring physical links in the network.
The FDDI standard calls for two counter-rotating rings with the secondary ring serving as a backup in case of a line fault in the primary ring. The physical connection layer PMD provides two pairs of connections to the network fiber optic cables: Primary-In/Primary-Out and Secondary-In/Secondary-Out. To implement a dual-ring configuration, interface equipment includes a receiver for receiving input signals PI from the primary optical ring cable and a transmitter for sending output signals PO to the primary optical ring cable as well as a receiver for receiving input signals SI from the secondary optical ring cable and a transmitter for sending output signals SO to the second optical ring cable.
The FDDI standard specifies two types of stations, or nodes: dual attach and single attach stations. Dual access stations (DAS) attach directly to both the primary and to the secondary rings of the FDDI network and take advantage of the extra 100 Mbits per second bandwidth of the secondary ring by using a dual MAC architecture, or system configuration.
On the other hand, single attach stations (SAS) connect only to a single ring by means of a concentrator. A concentrator is a special dual attach station that not only attaches to the dual ring but which also has multiple ports to facilitate a physical star network topology.
Redundancy of information paths is a very important consideration in designing an FDDI station to handle various cable and equipment fault conditions. All dual attach stations repeat information on both rings. Consequently, certain stations that offer key services to a network, such as file servers, are preferably dual attach stations to take advantage of their redundant transmission ability.
Mobility, on the other hand, is an important consideration for connection of computer workstations or personal computers that change location. Single attach stations are useful in an environment, such as an office, where mobility is an important system design consideration. Concentrators for connection of single attach stations to the network serve to shield the network when a nomadic station is disconnected and also verify that each single attach station is operating properly upon reconnection of a station.
A single attach FDDI station has a single MAC combined with a single PHY/PMD pair. A dual attach station includes a minimum of one MAC and two PHY/PMD pairs. By using a dual MAC a dual attach station can take advantage of the extra bandwidth provided by the secondary ring.
A variety of local area network topologies are obtained with the two different types of FDDI stations. For example, to support a tree topology, a single attach wiring concentrator provides second-tier connections to a network. Additional tiers, or levels, are introduced by connection of another concentrator to the second-tier connection, and so on.
The FDDI standard specifies that a dual attach station operates in various modes to accommodate the network to reconfigure itself in response to a fault condition in one of the optical cables or in one of the stations on the network.
An obvious fault occurs when the electrical power is removed from an FDDI station, or node. In that case, an FDDI node is equipped with optical by-pass relays which channel primary-ring optical signals from the primary optical input terminals directly to the primary-ring optical output terminals. Similar optical by-pass relays are used for the secondary optical ring. The FDDI standard permits for up to three consecutive stations to be optically bypassed.
A desirable feature of an FDDI station would be the ability to reconfigure the station to accommodate various cable link and node fault conditions.